Hum elimination system



Aug. 14, 1934.

w. VAN B. ROBERTS 1,969,902

HUM ELIMINATION SYSTEM Filed OGC. l1, 1930 ny, i

f i @um 2 Sheets-Sheet l INVENTOR WALTER VAN B.ROBEFLT5 ATTORNEY Aug; 14, 1934. w. VAN B. ROBERTS 1,969,902

HUM ELIMINATION SYSTEM Filed oct. 11M, 1930 2 sheets-sheet 2 INVENTOR WALTER VAN I B. ROBERTS ATTORNEY Patented Aug. 14, 1934 HUM ELIMINATION SYSTEM Walter van B. Roberts, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Y Application October 11, 1930, Serial No. 487,946

.5 Claims. (Cl. Z50-27) The present invention relates to hum elimination, and more particularly to novel and improved methods of, and means for, minimizing and eliminating hum pulsations in receiving circuitsl utilizing electron discharge tubes including screen electrodes, the electrodes of which tubes are energized from pulsating current sources.

It has been recognized in the prior art that when a receiving circuit, employing electron discharge tubes powered from commercial alternating current lighting mains, is used to receive broadcast signals, disagreeable hum notes arise due to the fundamental frequency of the alternating current source, as well as various harmonics thereof, these hum frequencies having been found to exist in the raw alternating current fed to the cathodes, and the rectified, pulsating current fed to the remaining electrodes.

While there may be some hum produced by the anode voltage ripples in the tubes used in the radio stages, such effects as there exist are practically negligible, particularly when loose coupled transformers are utilized between radio stages, for the reason that these loose coupled transformers do not transmit audio frequency currents impressed upon the primary of each radio transformer. As a consequence, the anode voltage ripples in the radio stages do not have any direct effect, except that there mightbe a slight tendency to cause modulation of the radio frequency signal current. However, this modulation may usually be ignored as its effects are easily kept small compared to the hum arising in the detector and audio stages and loudspeaker.

It is the main object of this invention to devise novel methods of, and means for, obtaining a complete balance between all hum producing factors.

Another important object of the invention is to provide a method of eliminating hum from the output circuit of an electron discharge tube provided with a control electrode, cathode, screen electrode and anode, and having the said electrcdes energized from a source of pulsating current, the method consisting in impressing voltages of frequencies similar to the hum producing frequencies upon the control electrode, and maintaining said iinpressed voltages at such a phase and magnitude as to neutralize all hum frequencies produced on said electrodes.

Another object of the invention is to provide a method of, and means for, balancing out hum of a given frequency from the acoustic output of an electro-acoustic system, particularly where said hum is produced by alternating current excitan tion of said system, the method consisting in imy n vpressing the alternating current upon a rectifier,

splitting the phase of the current of the hum frequencyr in the output of said rectifier, combining adjustable amounts of thel two phase rvoltagesof hum frequency derived from said split phase currentsapplying the resultant voltageV to the electro-acoustic system, and adjusting the magnitudes of the component two phase voltages to completely neutralize the hum of the given lfre- 85 quency first mentioned.

Still another object of the invention is to provide a self-contained unit for producing voltages of adjustable phase and magnitude and of any number of harmonic frequencies of an alternating current source, for substantially eliminating the effect of such harmonics from'the acoustic output of an electro-acoustic system connected to said source.

Still other objects of the invention are to improve generally the operation'and efficiency of radio receivers, and to particularly provide for broadcast receivers, powered from an alternating current source, reliable and economical arrangements for eliminating hum disturbances'of one, 80 or more, harmonics of the fundamental of said source.

The novel features which `I believe to becharacteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawings in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawings,

Fig. 1 diagrammatically shows a radio receiver embodying the present invention,

Fig. 2 diagrammatically shows a modified form of the invention,

Fig. 3 diagrammatically shows a self-contained unit embodying another form of the invention,

Fig. 4 shows the method of applying the mod 10g iication in Fig. 3.

Referring to the accompanying drawings in whichlike characters of reference indicate the same elements in the different figures, there is shown in Fig. 1 a receiving circuit comprising an 105 antenna circuit A, grounded as at G, and connected in any desired fashion to a selector and radio frequency amplifier circuit conventionally represented. It is not believed necessary to show the latter in anyl detail, not only for the reason 1.10

that the input electrodes of this tubeare coupled.. to the output circuit of the detector and ampli` fier circuit by any desired*conventional:type.v of coupling. i

The tube 1 is preferably of the type employing a screen electrode 2, the input circuit ofthe tube including a fixed condenser 3 in series with the control electrode 4, a grid leak resistor-5 being connected between the electroder4 and a resistor A37', the function ofthe latter'beingexplained later in detail. The anodeis connected, through a coupling resistor '7, to av point 9 on the rectiiier output circuit.r The Alatter includes a resistance 2;, '10, one terminal of which is connected by a lead 11 toi-the midpoint of the secondary 12 of the powervtransformer 13, and is also grounded. The other terminal of the resistorlO is'connected by a lead' 14 to the midpoint of the secondary 16 of the power transformer, or, in other words, to the hot cathode of the double wave' rectifier tube 15.v

As is well known to those skilled in the art, the Various electrode circuits of the detector tube, and other tubes of the receiver, are energized from a source E, the latter being a source of vcommercial alternating'current, such as the lighting power line, the source E being connected, through the power transformer 13 to the various tube electrode circuits. The ytransformer 13 usually includes three secondary windings, the winding 16 being connected to the cathode -17 of the rectifier tube 15,`the secondary 12 being connected between theanodes 18 of the rectifier tube, while the remaining secondary winding, not shown for simplicity of description, is connected to the primary windings, also not shown, of the cathode transformers of 'all tubes. The 'secondary 23, of the transformer associated with the detector tube 1, is connected between the terminals of the'heater element 24 of tube 1, the cathode 24 thereby being indirectly heated to emit electrons, as is well known to those skilled in the art.

The rectified output of they rectifier tube 15 .is filtered by the filter circuit which includes the resistor 25', shunt capacities 25,' and the .field coil 26 of the dynamic speaker 26', conventionally shown. The screen electrode'2 has positive potential impressed upon` it,'through"a lead 28 connected between it and the point 9 on the lead 14. The tube 1', in the rst audio frequency amplier stage has its control electrode 'connected to the high potential' terminal of the resistor '7, through a capacity 7', the low potential terminal of the resistor being connected, by a lead 8, to the point 9 on lead 14. The vanode'of tube 1' is, also, connected to point 9, through 'a lead 8', the latter including the primary coil'18' of the power vtransformer 19. It is to be understood that more than one stage of audio frequency amplifica- `tion Vcan Abe employed. f

l The cathode of tube 1' is heated from the source E,asexplainedheretofore the control electrode of-.the tube being biased` by a. resistor 20 'connected to the cathode as conventionally shown,

and in a manner well known to those skilled in the art. The voltage supply for the screen electrodes of the tubes in the radio frequency stages (not shown) is secured from the slidable contact which taps the resistor 10.

There will now be described the means provided for carrying out the `main purpose of the invention, the system as described so far being merely a well known form of radio receiver. In Fig. 1 the invention is practiced by impressing oni the detector grid 4 a potential of 120 cycle .frequencywhich is adjustable both in magnitude and phase.' Adjustment may, therefore, be made 'to neutralize completely any 120 cycle hum that maypbe'ipresent. inthe speaker 26'.

The rectifier output contains large amounts of 120 cycle current. .This current flowing through the'primary coil 51,' in" lead 14, produces a voltage. drop across potentiometer 37', connected across the secondary ccil 52, which voltageis nearly 90v degrees out-of phase with the primary current,fif`the transformer is suitably designed with this endy in view `in accordance with well known principles. The same current,also flows vthrough the resistor 51", in series'with coil 51, coil 26'and resistor 25', producing a voltage drop in phase with the current,`-and this drop is irnpressed on potentiometer 37, connectedV across the secondary coil 53, without rappreciablechange of phase by'means of transformer 54, 53.1`

rSincethe drops across potentiometer 37 and 3 7 `are outV of phase with eachother, if their midpoints are connected by'wire -55`, then, up to a certain: amount, difierencesof potential, between' sliders 36 and 36', may-Abel obtained rhaving an-y` desired phase and magnitude by suitable adjustment of the sliders. The slider 36', as already stated above, is connected by a lead 40 to Ythe gridv 4 of detector tube 1, while the slider 36 is connected by a lead 40 to the cathode 24.

So far only the 120 cycle hum hasfvbeen considered.' Due to the use of a full wave rectifier the'60 cycle current ripple in the rectifier output is negligible.'V Any 60 cycle hum arising from the use of raw 60 'cycle currention -the'tube cathodes 120 may be'eliminated in the usual fashion by proper location of taps 41 on the cathode heating Windings; The 'use of the full wave rectifier also substantially eliminates currents of-180.cycles from the rectifier output.

It is, however, possible that currents of vappreciable magnitude at 240 cycles, or more, may be present in the rectifier outputl and produce disturbing voltages onthe detector grid. Such voltages maybe reduced byv causing the rectifier outputcurrent to pass through a low pass filter befor entering elements 51, 51', but a more economical method is to insert a low pass filter (passing cycles, but not higher) in the line connecting sliders 36, 36' to ground and grid of the detector. 'The reason this alternative is preferred is that the'current in this line is negligible,

nso that the'series elements of the filter can be made comparatively cheaply. 1

It is 'obvious that the compensating potential 'produced bythemeans described. may be applied at'any'point in the'V circuits of the detector,.or

of'voltage will produce the desired effect, such as'the detector grid in Fig. 1.

'The present invention is not, limited tothe elimination of 'hum'of av particular `flrequency.. e

Figx2 shows any extensionbf therinvention to@gg include any number of the harmonics of the 60 cycle fundamental. For the sake of simplicity, however, only three have been shown. The details of construction and operation follow. Across the rectifier output, the rectifier and source E being conventionally shown, is shunted a circuit consisting of a condenser 50, an inductor 51 and a resistor 51', all in series. This circuit is tuned to the highest of the frequencies to be eliminated, in this case the frequency being 240 cycles. The resistor is of comparatively small value so that this circuit is traversed chiefly by current of 240 cycles, the voltage drop across the resistor 51 and one of the reactors being transferred to the potentiometer 37', 37 by meansof the transformers 51, 52 and 54, 53. Any desired phase and magnitude of 240 cycle voltage is derived from three potentiometers by means of sliders 36, 36', just as in Fig. 1.

A series inductance L1, in the filter system, is included to prevent the passage of 240 cycle current. Beyond this inductor L1 another tuned circuit is shunted across the filter, this one tuned to 120 cycles, having similar phase and magnitude adjusting means for the potentials at 120 cycles derived therefrom. Another series inductance L2 is followed by a 60 cycle tuned shunt circuit of similar nature to the others; the 60 cycle circuit being followed by another series inductance L3. The voltages of frequencies 60, 120 and 240 cycles derived from the three tuned circuits are added in series by series connection 60 of the pairs of sliders, and the total voltage is then impressed upon the detector or audio system, preferably a point where it will have the greatest effect. Adjustment of the six potentiometer sliders will then enable all hum of 60, 120 and 240 cycle frequencies to be eliminated.

In Fig. 3, there is shown a self-contained unit for producing voltages of adjustable phase and magnitude, and of any number of the harmonic frequencies of an alternating current source. As in the case of Fig. 2, the 60 cycle source E supplies the circuit, and the total voltage output is taken between the sliders 36, 36 connected by the series connections 60. 'Io secure the 60 cycle compensating voltage, in this form of the invention, compensating circuit constructed as disclosed in Figs. 1 and 2, is shunted across the secondary 61 of the transformer 62.

The 120 and 240 cycle paths, constructed as in Fig. 2, are shunted across the rectifier 63, the latter being in series with the secondary 64. Each compensating shunt path is tuned to a different harmonic of the fundamental frequency, the compensating fundamental being obtained, preferably, independently of the rectifier, as shown. A conductive path, generally denoted in Fig. 3 by the expression D. C. Load, is supplied across the last tuned, shunt path, to prevent the development of voltage across the condensers, such voltage preventing further electronic flow through the rectifier.

The method of utilizing the self-contained unit of Fig. 3 to cure hum in a radio receiver is disclosed in Fig. 4l. The detector stage of a conventional radio receiver is shown, elements not essential to the understanding of the use of the hum-eliminating unit, being omitted, it being clearly understood that all the electrodes of tube '70 are supplied from the source E, ina manner similar to that shown in Fig. 1.

The connection of the radio frequency coil 71 to ground is broken, and the output terminals 72, 72 of the unit of Fig. 3, connected therebetween. A radio frequency by-pa'ss condenser 73 is connected across the break. 'Ihe resultant effect is believed obvious from the aforegoing description, it being pointed out that the terminals 72, 72 can, also, be inserted in an audio frequency tube grid leak, or in a screen grid lead.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. The method of balancing out hum of a given frequency from the output of an electro-acoustic system where the hurn is produced by the alternating current component of imperfectly filtered fluctuating direct current which includes the steps of splitting the phase of the alternating current component causing the undesired hum combining adjustable amounts of the two phase voltages of hum frequency derived from said split phase currents, applying the resultant voltage to the electro-acoustic system and adjusting the magnitudes of the component to phase voltages to completely neutralize the hum of the given frequency first mentioned.

2. In combination a receiver comprising at 105 least one space discharge device having anode cathode and grid electrodes, a source of fluctuating direct current, means including a filter circuit for connecting said source of fluctuating current to at least one of said electrodes, a circuit 110 including phase splitting means interposed between the source and the filter circuit for deriving from said fluctuating direct current an alter- ,nating current component and means for transferring said derived alternating current component to another of said electrodes of said space discharge device, said phase splitting means including current phase and amplitude adjusting devices.

3. In a power supply system a source of fluctuating direct current, a filter circuit including means interposed between said source and said circuit for deriving from said fluctuating direct current alternating current components of various frequencies and means for controlling the phase and amplitude of each current at said various frequencies.

4. In combination, an amplifier comprising an lelectronic device, means for energizing said electronic device comprising a source of fluctuating direct current and a filter circuit connected to said device, means intermediate the source and filter for deriving from said fluctuating current alternating current of a desired frequency, means for controlling the phase and amplitude of said alternating current and means for impressing said alternating current upon said electronic device.

5. In combination with a space discharge device circuit space current supply means comprising a source of uni-directional fluctuating current, means coupled to said source for deriving from the uni-directional fluctuating current at least two alternating currents of the same frequency but of differing phase, variable means for deriving from said two alternating currents a single current of predetermined phase and amplitude and means for impressing the last named derived single current upon the input electrodes of said space discharge device.

WALTER vAN B. ROBERTS.

DISCLAIMER 1,969,902.Walte1 Van B. Roberts, Princeton, N. J. HUM ELIMINATION SYSTEM. Patent dated August 14, 1934. Disclaimer filed September 13, 1935, by the assignee, Radio Corporation of America.

Hereby enters this disclaimer to that part of the claims in the specification of said Letters Patent which is in the following words, to wit:

1. The method of balancing out hum of a given frequency from the output of an electro-acoustic system where the hum is produced by the alternating current component of imperfectly filtered fluctuating direct current which includes the steps of splitting the phase of the alternating current component causing the undesired hum Combining adjustable amounts of the two phase voltages of hum frequency derived from said split phase currents, applying the resultant voltage to the electroacoustic system and adjusting the magnitudes of the component to phase voltages to completely neutralize the hum of the given frequency rst mentioned.

5. In combination with a space discharge device circuit space current supply means comprising a source of uni-directional fluctuating current, means coupled to said source for deriving from the uni-directional iuctuating current at least two alternating currents of the same frequency but of diering phase, variable means for deriving from said two alternating currents a single current of predetermined phase and amplitude and means for impressing the last named derived single current upon the input electrodes of said space discharge device.

[Oficial Gazette October 8, 1935.] 

